A power driven ski-bob and ski are newcomers to the field of winter recreation. Presently there is a polarization between outdoors winter sports enthusiasts. The purists adhere to skis propelled by gravity or skier's muscles, while the modernists enjoy mounting a snowmobile which employs brute force to propell itself and its riders. Power driven ski devices (shown in U.S. Pat. Nos. 3,853,192, 3,966,010 and co-pending U.S. patent application Ser. No. 754,457 which are herein incorporated by reference), bridge this gap between the purists and modernists. They do so by combining the excitement of using one's sense of balance to control and steer with the excitement of controlling one's propelling power, and they remove or reduce the objectionable aspects of snowmobiling. For example, the total weight of a current power driven ski-bob is around 30 pounds and it is propelled by a gas thrifty 3 HP engine, versus a snowmobile which weighs several hundreds of pounds and uses a gas guzzling engine ten times more powerful. Therefore, in contrast to snowmobiles, power driven ski-bobs and skis have a minimal effect on the trail they pass, they do not develop large kinetic energy that may endanger their rider and others and they are not likely to get stuck in the snow, since the average rider can lift a unit in one hand. The small engine generates less acustical energy which can be readily muffled to non-obtrusive levels. Further, power driven ski-bobs and skis are characterized by their long and narrow footprints, and have no stability of their own. Thus, the rider has to continuously maintain balance, which makes riding an involving and exciting experience.
At this point it may be helpful to briefly discuss some of the relationships between the tread's effectiveness to the weight and power of a snow going device:
When a propulsion system is less effective in engaging snow and developing thrust against it, it has to be made to cover a larger area, which in turn requires a larger engine and chasis, which in turn require an increased thrust to be moved, especially uphill. In addition, a bulky tread requires a substantial power just to be circulated in the snow. Thus, the present snowmobile weighs and requires as much power as it does. In contrast, it is an object of the present invention to provide a light, compact and effective propulsion system that would effectively translate the power of a small engine to thrust and enable the construction of light snow going devices such as power driven ski-bob and ski weighing as little as 30 and 15 pounds respectively.
Another aspect of designing a snow going device is that snow can be anything from rock hard to a fluffy powder, it can be a fraction of an inch or several feet deep. In addition the coefficient of friction between the same snow and the same ski runner surface may increase ten fold with temperature drop (Please refer to an article entitled "Properties of Snow" published in "Cold Regions Science and Engineering", December 1964, by Mr. Malcolm Mellor of the U.S. Army Material Command, Cold Regions Research & Engineering Laboratory, Hanover, N.H., for further data on coefficient of friction between snow and typical ski runner materials and wax coatings). Thus, present snowmobiles require several square feet of contact area between the tread and the snow and a large engine coupled to it, usually through a transmission, to adapt to the extreme conditions that combinations of these factors can produce. In contrast, it is an object of the present invention to obtain sufficient traction to propel one or two people uphill, over various snow conditions, with a compact propulsion system having a contact area with the snow as small as 1/2 a square foot.